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John Garry reported 'sdebug_q_cmd_complete: scp is NULL' failures that were mainly seen on aarch64 machines (e.g. RPi 4 with four A72 CPUs). The problem was tracked down to a missing critical section on a "short circuit" path. Namely, the time to process the current command so far has already exceeded the requested command duration (i.e. the number of nanoseconds in the ndelay parameter). The random=1 parameter setting was pivotal in finding this error. The failure scenario involved first taking that "short circuit" path (due to a very short command duration) and then taking the more likely hrtimer_start() path (due to a longer command duration). With random=1 each command's duration is taken from the uniformly distributed [0..ndelay) interval. The fio utility also helped by reliably generating the error scenario at about once per minute on a RPi 4 (64 bit OS). Link: https://lore.kernel.org/r/20200813155738.109298-1-dgilbert@interlog.com Reported-by: John Garry <john.garry@huawei.com> Reviewed-by: Lee Duncan <lduncan@suse.com> Signed-off-by: Douglas Gilbert <dgilbert@interlog.com> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.